1. Field of the Invention
This invention relates to a downhole digital power amplifier, and more specifically a device which, through digital means, provides a sinusoidal, variable frequency, variable power and phase-shift modulated output for the transmission of data from a downhole sensor arrangement to a coupling transfer device, or the like. Selection of and variation for frequency, power and modulation are made by digital inputs, and the device provides these functions at extra low frequencies (ELF), low-to-high power output, and varying phase-shift keying. Although the subject invention has a range of applications, the invention has particular utility as a downhole digital power amplifier for use in a measurements-while-drilling (MWD) system.
The incentives for downhole measurements during drilling operations are substantial. Downhole measurements while drilling will allow safer, more efficient, and more economic drilling of both exploration and production wells.
Continuous monitoring of downhole conditions will allow immediate response to potential well-control problems. This will allow better mud programs and more accurate selection of casing seats, possibly eliminating the need for an intermediate casing string, or a liner. It also will eliminate costly drilling interruptions while circulating to look for hydrocarbon shows at drilling breaks, or while logs are run to try to predict abnormal pressure zones.
Drilling will be faster and cheaper as a result of real-time measurement of parameters such as bit weight, torque, wear and bearing condition. The faster penetration rate, better trip planning, reduced equipment failures, delays for directional surveys, and elimination of a need to interrupt drilling for abnormal pressure detection, could lead to a 5 to 15% improvement in overall drilling rate.
In addition, downhole measurements while drilling may reduce costs for consumables, such as drilling fluids and bits, and may even help avoid setting pipe too early. Were MWD to allow elimination of a single string of casing, further savings could be achieved since smaller holes could be drilled to reach the objective horizon. Since the time for drilling a well could be substantially reduced, more wells per year could be drilled with available rigs. The savings described would be free capital for further exploration and development of energy resources.
Knowledge of subsurface formations will be improved. Downhole measurements while drilling will allow more accurate selection of zones for coring, and pertinent information on formations will be obtained while the formation is freshly penetrated and least affected by the mud filtrate. Furthermore, decisions regarding completing and testing a well can be made sooner and more competently.
There are two principal functions to be performed by a continuous MWD system: (1) downhole measurements, and (2) data transmission. The subject invention pertains to an element in the data transmission aspect of MWD.
2. Description of Prior Art
The transmission of data or other information from downhole sensors or telemetry systems has typically incorporated such techniques as (1) mud-pressure pulse, (2) insulated conductor, (3) acoustic generation, and (4) electromagnetic waves. These techniques utilize an analog input and analog circuitry to provide the desired frequency, power and modulation outputs. The use of analog devices and techniques has proven in the past to be bulky and relatively inaccurate, particularly at extra low frequencies and high power. Because of this, transmission rates and frequencies are normally set at a single value.
In a mud-pressure pulse system, the resistance to the flow of mud through a drill string is modulated by means of a valve and control mechanism mounted in a special drill-collar sub near the bit. The communication speed is fast since the pressure pulse travels up the mud column at or near the velocity of sound in the mud, or about 4,000 to 5,000 fps. However, the rate of transmission of measurements is relatively slow due to pulse spreading, modulation-rate limitations, and other disruptive limitations such as the requirement of transmitting data in a fairly noisy environment.
Insulated conductors, or hard-wire connection from the bit to the surface, is an alternative method for establishing downhole communications. The advantages of wire or cable systems are that: (1) capability of a high data transmission rate is provided; (2) power can be sent downhole; and (3) two-way communication is possible. This type of system has at least two disadvantages; it requires a special drill pipe, and it requires special tool-joint connectors.
To overcome these disadvantages, a method of running an electrical connector and cable to mate with sensors in a drill-collar sub was devised. The trade-off or disadvantage of this arrangement is the need to withdraw the cable, then replace it each time a joint of drill pipe is added to the drill string. In this and similar systems the insulated conductor is prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string. Also, cable techniques usually entail awkward handling problems, especially during adding or removing joints of drill pipe.
In addition, hardwire systems use high frequencies which utilize smaller components to circumvent size problems downhole. While the data rate is higher, the number of analog components required is great. In addition, in acoustic-type systems, an acoustic (or seismic) generator is located near the bit. Acoustic methods use the higher frequencies which are not only affected by acoustic noise during drilling, but also by higher signal attenuation through the conducting media. Accordingly, it is considered highly desirable to develop a transmitter which will occupy a small area, generate frequency in the ELF range at high power, and yet be capable of modulating data at a moderate rate with comparatively low loss. Power for this generator would have to be supplied downhole. The very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, make signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compound the signal attenuation problem for drill-pipe transmission. Moreover signal-to-noise limitations for each acoustic transmission path are not well defined.
Finally, the last major previously known technique comprises the transmission of electromagnetic waves through a drill pipe and the earth. In this connection electromagnetic pulses carrying downhole data are input to a toroid positioned adjacent a drill bit. A primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is provided by the drill pipe. A receiver is connected to the ground at the surface and the electromagnetic data is picked up and recorded at the surface.
Downhole arrangements and devices known in the prior art are typified by Zill et al--U.S. Pat. Nos. 3,618,001 and 3,750,098 which disclose a downhole acoustic logging control system, wherein a single channel is arranged to provide at least two degrees of amplification of electrical signals in a sequence synchronized with transmissions of acoustic energy from the transmitter. In such an arrangement, control of gain selection originates in the downhole device, and there are no digital commands. More specifically, as shown in FIG. 3 of the aforementioned patents, a mechanical gain select relay and associated mechanical contacts are controlled, via a mode control unit, by a sequence counter. In such an arrangement, the varying degrees of amplification of the variable gain amplifier may only be chosen in a fixed sequence (for example, low, medium, high gain). Thus, such an arrangement is burdened by the disadvantages of inflexibility in designating a particular gain value (as is accomplished, for example, by the use of a digital command).
Another arrangement in the prior art is disclosed in Baldwin et al--U.S. Pat. No. 3,518,679. That patent discloses a well logging system employing a three-conductor logging cable for transmitting signals and power between the surface and an acoustic logging tool. As seen in the patent, the downhole arrangement has an amplifier and gain control system controlled from the surface by a switch system so as to switch gain values between certain predesignated values. The disclosed arrangement also includes a downhole switch controlled by an uphole switch for selecting an orienting output signal of the circuit or a rotating switch signal of the circuit in the downhole device, such signals being selected for transmission to the surface. It is to be emphasized that only certain gain values may be selected by connecting a switch line to respective contacts under the influence of a solenoid. Thus, the arrangement of the subject patent is again burdened by the disadvantages of lack of flexibility and lack of capability of designating precisely the particular gain value selected.